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ANNOUNCER: This is CNN breaking news. AARON BROWN, HOST: Well, good evening again. As we told you a few moments ago, we're going to go live to Washington, D.C. and a press conference there where earlier today, this afternoon some mail found in the mailroom of the Senate majority leader tested positively in the initial test for the compound ricin, which is very dangerous stuff and deadly stuff.

These first tests are not always accurate. We learned that from anthrax. And we're reminded of that again today.

This is the capital police and also the majority leader, Senator Frist.

SEN. BILL FRIST (R-TN), MAJORITY LEADER: ... Bill Frist, majority leader of the United States Senate. And over the next several minutes, we'd like to give an update on the findings and the occurrences today in the Dirksen Building of the United States capital.

You will be briefed by the chief of the capital police, Chief Terry Gainer, and I'll be happy to respond to the medical aspects of what has happened today.

Chief Gainer.

CHIEF TERRANCE GAINER, CAPITOL POLICE: Thank you, Senator.

Just after 3 p.m. today, the United States Capitol Police was notified by a postal worker in room 454 of the Dirksen Building of a suspicious white powder. Our hazardous device unit responded there and did see some powder. It is a large room, used to handle mail.

It was not clear what package or what letter this powder may have come from, but it was at least suspicious.

They conducted two tests. One test came back positive for ricin, but the operators of that thought it was a false positive. They then did a second test which came back negative.

Following standard police protocol, they took the samples of that to another facility we had on the campus, where there more tests were done. Each one of those tests indicated positive for ricin.

As those tests were concluded, we then shut down the air handlers to these buildings, in order to control the environment and began the process of getting people out of that fourth floor hallway along the south side of the building, Constitution Avenue.

Simultaneously with that, we were then taking these samples to a lab off the Hill, where it would be done in a very professional lab type environment. We've been waiting for the conclusion of those results, and we have those back in. Two of those three tests do indicate that it is ricin.

So we have had several confirmations that it is ricin. With that, then, we've undertaken a procedures of decontaminating the -- some 16 people who were on the floor at that time, when the first positive came through. And working with them and their families to get them back home. And then reach out to anybody else who may have been in that area.

There will be one further test done that we don't expect the results back until the early morning hours of tomorrow, which will give us some indication on what our next steps are with this particular building or on the campus.

So at the moment, we're in a wait and see position from an analytical point-of-view in what next steps we may take -- Senator.

FRIST: Thank you, Chief Gainer.

First of all, most importantly, nobody's been hurt, and everybody is fine. There have been absolutely no injuries whatsoever...

BROWN: The senator is also a doctor, and that may help as he starts talking about the medicine of all of this.

FRIST: ... a poison, a toxin. This is a criminal activity, and this, as Chief Gainer said, will be investigated as such.

And to all the families, everybody is fine.

Ricin, it is important for people to understand, is a poison. And it is a toxic poison. It is not contagious. That is, it cannot be easily spread from one individual to another.

It is a poison, a toxin. It is derived from a castor bean. And a lot of people around the world ingest castor beans. But it is refined in such a way it becomes very toxic.

We are most concerned about the inhaled. The powder that was found could theoretically have been inhaled. There's been no evidence of that by any of the people in the office or on the floor.

The symptoms, typically, would appear from anywhere four to eight hours. We're beyond the eight hours of exposure. Again, everybody is just fine.

The symptoms, it is important -- If there were people on the fourth floor of the Dirksen today, if any of them did have any symptoms of shortness of breath, or chest tightness or a cough that had come up over the last several hours, it would be important for them to notify the physicians, the capital physician's office, a number which we will give you here shortly.

With that, there is no cause for alarm. It does mean that as the test results come in over the course of the night, certain decisions will be made about either offices closed tomorrow or parts of the floor closed tomorrow, or parts of the buildings. We would expect to make an announcement about that at 5 a.m. in the morning, again, as the test results come back.

I do want to commend the response, as we've seen it today, the rapid response, the testing that took place and the timely factor, the excellent response by the police.

It's been a trying day, because we've had a number of people who are -- are -- and have been inside. They are going to go through a very simple, straightforward, routine decontamination process. And then they'll be able to go home a little bit later tonight.

With that, Chief, anything else to add at this point? If not, we'll turn to questions.

GAINER: No, sir.

FIRST: We'll be happy to take any questions. I'll refer anything about the investigation to the chief, or anything about the medical aspects, I'll be happy. Let me start with the question in front of me.

UNIDENTIFIED MALE: How serious is ricin as a bioterrorist agent?

FIRST: The question is, how serious is ricin as a bioterrorist agent? And I would regard this as a bioterrorist agent. Most of you have heard of ricin.

Let me say that no -- to the best of my knowledge, and -- and I don't want to be held to this -- but to the best of my knowledge, in human beings, ricin has never caused serious injury.

And I say that, really, just to -- to sort of affirm what I said before. Nobody is sick. We don't expect anybody to get sick, although we know it is a very toxic chemical toxin. It's a poison. We know that in animal models or in rat models that if this agent, ricin, which it can be a powder, or it can be injected as well. But if it is inhaled by animal models, it's very toxic to the lungs and can cause a -- a dysfunction of the lungs, a shortness of breath or a building up of fluid in the lungs.

But today, to the best of my knowledge, there has been no evidence of that being used in humans. There has, in the 1970s most of you will recall there was a particular case where ricin was injected, given what we call parentally (ph). This is very different. And then, that had a devastating effect.

BROWN: Senate Majority Leader Bill Frist, it was mail coming to his area, not necessarily his specific office.

Jeanne Meserve is with us. Jeanne tracks homeland security matters.

I'm just looking at my notes. It looks like they've done eight separate tests on this substance. Six of the eight came back positive. And we are waiting, I guess, for the most definitive test, which is going on, I gather, at Fort Deitrick?

JEANNE MESERVE, CNN CORRESPONDENT: ... at Fort Deitrick in Maryland. The Army there has a research lab where they're doing more extensive testing on this substance. They hope to have results tomorrow which will indicate whether or not this is indeed ricin or these are false alarms.

But certainly, the number of tests done so far, the number of positive results, would seem to indicate they -- they're handling this very seriously.

BROWN: And just -- just go back a second to how this all came out. This came out through the Department of Homeland Security?

MESERVE: Well, different aspects of it came out from different sources. The capital police informed us this afternoon that there had been an incident in the mail room on the fourth floor of the Dirksen Senate Office Building.

They responded it, too, at the time. My understanding from our staffer on the scene was that at about 4 p.m., they waved everybody off, saying there didn't appear to be anything to this.

Obviously, subsequent testing brought them to a different conclusion. I heard it from a source, a homeland security official, earlier tonight that the tests were positive for ricin. That of course, turns out to have been correct.

BROWN: All right. Jeanne, thank you for your efforts. Jeanne Meserve in Washington. As you could see around the capital, they are -- they are engaged, I guess, in investigating this. This is obviously, if it turns out to be ricin, a very serious, very serious matter. It takes us back a couple of years to the anthrax mailings, some of which ended up in the capital, some of which ended up in other places.

In the meantime, the tests go on. There are indications it's ricin but not -- not an absolute test at this point. We'll just have to wait for more information. That's the news at this moment.

And we will stop here, and most of you will them join "LOU DOBBS TONIGHT" in progress. Good night.

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